1. Field of the Invention
The invention relates to a magnet assembly for use in carrying out nuclear magnetic resonance experiments on a body or sample. The invention is particularly concerned with an assembly for use in carrying out liquid state nuclear magnetic resonance (NMR) measurements on hyperpolarised samples.
2. Description of Related Art
The widespread application of nuclear magnetic resonance! (NMR) has been hampered by poor sensitivity due to the very weak polarization of nuclear spins even in a strong magnetic field at room temperature (eg: 13C polarization is only 8 ppm at 9.4 T). A technique to increase the polarization of the sample by a factor more than 10000 has been disclosed by Ardenkjaer-Larsen et. al (PNAS Vol. 100, #18, p. 10158-10163, and patent application WO02/37132). This process involves hyperpolarising the sample and then dissolving it in a hot solvent, or melting it by direct application of heat, and then moving it rapidly as a liquid into a magnet where an NMR or MRI measurement is made. The <1> hyperpolarization process typically requires the sample to be cooled to a few Kelvin or below and exposed to a strong magnetic field.
A particularly versatile and effective form of hyperpolarization is Dynamic Nuclear Polarization (DNP). This involves the steps of:                mixing the sample with a source of free electrons (i.e., a free radical).        cooling the sample to typically ˜1.3K in a strong magnetic field (typ. 3.35 T), at which temperature the electron spins are almost fully polarized.        irradiating the sample with microwaves near the electron paramagnetic resonance (EPR) frequency, thus causing efficient transfer of electron polarization to the nuclear spin system. This process proceeds with a time constant of tens of minutes and typically results in a polarization of 30% 13C.        thermally isolating the sample from the source of cold.        rapidly melting or dissolving the sample in hot solvent whilst still in the strong magnetic field. There is no requirement for magnetic field homogeneity during the melting phase. The sample polarization after dissolution is typically ˜20%.        
The final step is carried out rapidly so that the sample temperature passes through the minimum in T1 (spin-lattice relaxation time constant) rapidly and retains a significant proportion of the hyperpolarization achieved by DNP. The sample now decays with characteristic T1 This is of the order seconds for 13C, so it is advantageous to move the sample as rapidly as possible to the NMR measurement magnet. The T1 of other species (eg: proton, 15N) are much shorter, so rapid transfer is of even greater importance.
Suitable apparatus to carry out the DNP process is described in detail in the references above. It suffices to say that the required elements of the DNP polarization cell within the DNP magnet are:                a means of cooling the sample to ˜1 to 1.5K, typically by immersion in a pumped liquid helium bath        a means of applying low power microwave radiation to the sample        a means of thermally isolating the sample from the source of cooling        a means of applying heat or hot solvent to melt and/or dissolve the sample        a means of transporting the sample out of the DNP cell        
Like NMR, DNP is a resonant technique, so the magnetic field should be uniform across the sample volume during hyperpolarization. The degree of uniformity is determined by the EPR linewidth of the free radical, and is typically of the order of a few parts per million, and no less than 100 ppm.
The apparatus disclosed in WO02/31732 uses separate superconducting magnets in separate cryostats to provide the suitable homogeneous magnetic field regions for DNP and NMR. The sample must traverse the distance between these magnets after melting/dissolution, during which time the hyperpolarization decays. The signal components arising from protonated carbons (eg: methyl groups) have shorter T1s of the order ˜1 s. The distance the sample must travel between separate magnets is a few meters, which typically takes 3-6 seconds (the sample may be carried in a container or flowed down a pipe). The loss of methyl polarization is therefore of the order:1−e(−3/1)=95.021%
It is therefore clearly desirable to reduce the distance between the DNP and NMR regions to reduce this signal loss.